The presence of double-stranded RNAs M1 or M2 in virus-like particles enables cells to secrete the K1 or K2 toxins and be immune to these toxins. We have defined 29 chromosomal genes (called makl, .. ) needed to replicate the M dsRNA and 5 regulatory chromosomal genes affecting this process (ski genes, KRB1). M2 is excluded by M1 dsRNA. Another plasmid, called [EXL], also excludes M2. Yet another, [NEX], prevents this effect of [EXL] on M2. A third, [HOK], acts as a helper for a replication-defective mutant of M1. Three very different L dsRNAs (L-A, L-B, and L-C), all the same size (3 x 10 to the 6th daltons), are present in various strains. [EXL], [NEX], and [HOK] are located on variants of L-A. L-B or L-C are stably maintained in the same cell as L-A. Virus-like particles in strains carrying only L-B or L-C have different proteins than those carrying L-A.